Inertia welding is a process used to join metallic articles which are generally symmetrical about an axis of rotation. Such articles may be solid or hollow. Thus for example inertia welding can be used to join components together to form an article such as a crankshaft or a welded hollow tube assembly. The inertia welding process is described for example in U.S. Pat. Nos. 3,234,644; 3,235,162; 3,462,826; 3,591,068; and 4,365,136 which are incorporated herein by reference.
Briefly, in inertia welding the articles to be joined are located and positioned so that their axes of symmetry coincide and the surfaces to be joined are in a parallel relationship. One of the articles is held stationary, the other is attached to a rotatable flywheel. The rotatable article--flywheel combination is accelerated to a predetermined rotational speed and the rotating article is then forced against the stationary article. The flywheel geometry, mass and rotational speed determine the available kinetic energy, and this kinetic energy is dissipated (converted into thermal energy) by friction between the articles to be joined. The articles are forced together and the dissipated kinetic energy is sufficient to cause localized softening. When the flywheel rotation stops, the force between the articles is maintained or increased causing the softened portions of the articles to bond together. The force between the articles causes plastic or superplastic deformation in the weld zone. Cooling of the weld zone is fairly rapid by conduction of heat into the articles.
The inertia welding process is conducted under conditions which cause expulsion of a significant amount of material from the weld zone thus inherently removing detrimental surface contamination. The weld zone is more characteristic of a forging rather than a casting. Weld zones produced by other forms of welding such as laser, electron beam and electric fusion welding have weld zones which have been melted and resolidified and, therefore, have the characteristics of a casting which are generally less desirable than the characteristics of a forging which are approximated by inertia weld zones.
Inertia welding is a form of friction welding. Another form of friction welding relies on a continuous motor drive to provide frictional heating rather than flywheel stored energy. As used herein the term inertia welding includes other forms of rotational friction welding.
Inertia welding was developed and has been widely used in joining ferrous materials such as iron and steel in the heavy construction equipment industry. Recently it has been employed with reasonable success in joining superalloys. The joining of superalloy materials is much more demanding than the joining of ferrous materials since superalloys have higher softening temperature and are much more resistant to high temperature deformation. Inertia welding of "powder processed" superalloys are the most difficult of all inertia welding applications. The zone in the articles to be joined, which is to be softened by the welding process, is limited and the degree of upset or deformation in the weld zone is similarly limited. Consequently, in the inertia welding of superalloys (and particularly powder processed superalloys) there is generally a residual notch observed at the weld zone. Such a notch is not often observed in inertia welding of ferrous materials.
Unfortunately, in the case of powder processed nickel superalloys the weld zone notch invariably extends inwardly beyond the original diameters of the welded components. Thus, even after the weld upset is removed by machining there a notch usually remains and removal of the notch requires machining to less than the original diameters of the articles which were joined. If the notch is not fully removed, it acts as a stress riser and as a failure initiation site during subsequent use of the welded article or even during the subsequent heat treatment. This notch problem is particularly detrimental in higher strength superalloys i.e. those having yield strengths in excess of 100 ksi at 1000.degree. F.) and in superalloy materials produced by the powder metallurgy techniques.
In the initial fabrication of superalloy articles by inertia welding the notch problem can be overcome, at some economic cost, by making the initial parts oversized and machining the welded assembly down to a size sufficient to eliminate the notch. Unfortunately this is not generally practical when it is desired to repair a damaged article by inertia welding. This is because the undamaged portion of the article has already been machined to a particular diameter, usually a minimum design diameter which cannot be reduced without adversely weakening the part. Thus, when attempts have been made to remove a damaged portion of a superalloy article and replace it with a new portion, the inertia weld has been the weak point in the finished article because of the weld zone notch which either acts as itself to weaken the article or requires that the article be machined undersized.
Even in the original fabrication situation the use of oversized articles may exceed the capacity of available inertia welding machines.
There are also weld geometries which have heretofore been difficult to inertia weld. One of these problem geometries is encountered in producing tapered articles such as hollow cones. This problem is encountered in joining a shaft to a disk in a gas turbine engine.
Accordingly it is an object of the invention to describe a method for inertia welding materials and controlling the depth and location of the weld zone notch.
It is another object of the invention to disclose a method of inertia welding high strength (and/or powder metallurgy processed) superalloys while minimizing the deleterious effects of the weld zone notch.
Finally it is an object of the invention to disclose a geometry for inertia welding articles together to form a conical hub.